1. Field of the Invention
The present invention relates to a magnetic field measurement apparatus consisting of a magnetometer comprised of a Superconductive Quantum Interferometer Device (SQUID) for measuring magnetic fields generated by the nervous activity of the brain of humans or animals or myocardial activity or magnetic substances contained in the subject to be inspected.
2. Description of Related Art
In measurements of very weak magnetic fields in the conventional art using equipment such as SQUID for measuring biomagnetic fields, generally the magnetic field on the surface of a living body is capable of being measured. Such measurements can be just the vertical components of a magnetic field with the head regarded as a sphere, for instance the polar coordinates (r, xcfx86, xcex8) in the case of the head, and the magnetic field component Br in the vertical r direction on the head surface, or in the case of the heart, the orthogonal coordinates (X, Y, Z), of the chest section when measured on the flat planes X and Y, and the magnetic field component BZ in the vertical Z direction on the X and Y planes.
On the other hand while few in number, there is literature reporting on measurement apparatus for measuring magnetic components of a biomagnetic field in a plurality of directions. For instance, the simultaneous measurement of the magnetic component BX in the X direction and the magnetic component BY in the Y direction on the orthogonal coordinates (X, Y, Z); as well as the display of magnitude {square root over ( )}(BX2+BY2) synthesized by magnetic component BX in the X direction and magnetic component BY in the Y direction have been reported (K. Tsukada et. al., Rev. Sci. Instrum., 66 (10), pp 5085-5091 (1995)).
Further, though not the three directions Br, Bxcfx86, Bxcex8 of the polar coordinates (r, xcfx86, xcex8) of the magnetic components BX, BY and BZ in the three directions of the orthogonal coordinates (X, Y, Z); a method has been reported for measuring the three components of each intersecting magnetic field, finding the magnetic components Br, Bxcfx86, Bxcex8 in the three directions on the polar coordinates (r, xcfx86, xcex8) and displaying a waveform showing the time variation of each magnetic component in three directions on polar coordinates (r, xcfx86, xcex8) on a CRT screen (Y. Yoshida et. al., 10th Int""l Conf. on Biomagnetisim (1996)).
Also, in the conventional art, not only a waveform showing time variations in a magnetic field strength, but also the distribution of the magnitude of a magnetic field can be found from results of magnetic measurements of a plurality of points in an organism utilizing a plurality of magnetometers, and the result displayed as a magnetic field magnitude contour map. Factors such as the position, magnitude and direction of electrical current sources in an organism can be analyzed over desired periods of time on a magnetic field contour map and changes over time in the electrical physiological phenomenon in the organism thus discovered. In the conventional art, changes in electrical physiological phenomena in a dynamic organism can therefore be revealed by utilizing these magnetic field contour maps to aid in the diagnosis of disease.
In the method used in the conventional art, the heart of the child or adult which is the subject of measurement is fixed in a constant position and direction versus the magnetic plane of the magnetic field of the magnetometer. However, there is the problem that when measuring the magnetic field of the heart of a fetus, an accurate measurement of the heart""s magnetic field cannot be made since the position and direction of the fetus cannot be fixed since the fetus is constantly moving within the body of the mother. In other words, even if there is no change in the electrical current source within the heart of the fetus, the position and direction will change versus the magnetic plane of the magnetic field generated in the magnetometer by the heart of the fetus, creating the problem that the time waveform and the components of the magnetic field being measured cannot be fixed. Another problem in the conventional art, is that a standardized waveform cannot be obtained due to variations in the magnetic field waveform due to changes in the body position of the fetus within the body of the mother, making an accurate diagnosis of the heart disease of the fetus difficult. Further, when the position of the Dewar""s vessel housing the magnetometer is moved in order to increase the magnetic signal to measure the component in just one direction of the magnetic field, the magnetic signal reaches a maximum and the measurement range narrows so that setting an ideal position and direction for measurement with the Dewar""s vessel is difficult, creating the problem that a long time is required. A still further problem is that a large drift occurs in the magnetic signal being detected when moving the Dewar""s vessel to an optimal position versus the subject being measured and a long time is thus required to stabilize the magnetic signal being detected.
Yet another problem is that high sensitivity non-destructive inspection of minute impurities having magnetic properties within a nonmagnetic substance is difficult, and furthermore the investigation cannot be conducted with high speed.
In order to resolve the above mentioned problems, it is therefore an object of this invention to provide a magnetic field measurement apparatus and a magnetic field measurement method for accurately measuring the electrical physiological phenomena within the heart of a fetus without affecting a change in the status of the fetus, even when the direction and position of the fetus changes within the body of the mother. It is a further object of the invention to provide a magnetic field measurement apparatus and a magnetic field measurement method for accurately detecting changes over time in the magnetic field from the subject of inspection, even in cases where a position change has occurred in the subject for inspection while placed in an environment for inspection or the subject for inspection is placed inside a special material for inspection.
The magnetic field measurement apparatus of this invention is comprised of detection coils for detecting magnetic fields of three directions and a superconductive quantum interferometer device (SQUID) connected to these detection coils; a single or a plurality of vector magnetometers are provided for isolating and measuring each of the magnetic components for the three directions. The magnetic components of the intersecting three directions measured with the single or plurality of vector magnetometers are synthesized by the square sum method and a time waveform of the resulting magnitude of the magnetic field is shown on a display means (monitor).
In the magnetic field measurement apparatus of this invention, a holding means and control means for storing and cooling the vector magnetometers, maintaining the vector magnetometers in a superconductive state in the Dewar""s vessel and varying the direction of the center position of the bottom of the Dewar""s vessel towards the subject for inspection is provided. The center position of the bottom of the Dewar""s vessel is set to an ideal position and direction versus the subject for inspection so that the magnitude of the time waveform reaches a maximum, while observing the time waveform on the monitor. When shifting the bottom of the Dewar""s vessel for optimum direction and position while watching the monitor, and the frequency band width of the magnetic field being measured is widened, time is required for the signal drift to significantly stabilize so that in order to remove the drift of the output waveform when moving the Dewar""s vessel, the signal from each magnetometer is split into two signals. One of these signals is fed to a highpass filter, the signal then processed and displayed on the monitor. The other signal is input to a data analysis and storage device such as a personal computer.
The magnetic components of the three directions are BX, BY, BZ of the three directions of the orthogonal coordinate system (X, Y, Z): are Br, Bxcfx86, Bxcex8 of magnetic components of the three directions (r, xcfx86, xcex8) of the polar coordinate system. The magnetic components Br, Bxcfx86, Bxcex8 are converted into the magnetic components BX, BY, BZ of the three directions. A coil bobbin made from highly polymerized insulating resin piece such as glass fiber reinforced epoxy resin, fiber reinforced plastic (FRP) or PEEK is mounted with detection coils in three intersecting directions and made from superconducting material for isolating and detecting the direction of magnetic fields having different mutually intersecting surfaces, and comprising a vector magnetometer for measuring BX, BY, BZ simultaneously. The center of the surface forming the three detection coils is provided along the center of the bobbin. Further, a surface forming one detection coil is perpendicular to the center axis of the coil bobbin. The surfaces forming the other two detection coils intersect and are also parallel to the center axis of the coil bobbin.
The intensity (magnitude) of the magnetic field vector expressing a synthesis of {square root over ( )}(BX2+BY2+BZ2) is found from the magnetic components BX, BY, BZ of the three directions simultaneously measured by using the vector magnetometers and shown as a magnetic field magnitude distribution graph on the display means. Changes over time of the intensity of the magnetic vectors are then monitored.
In this invention therefore, the magnetic components of the three intersecting directions formed by the electrical current source generated by the subject being examined are all simultaneously measured, a squared sum taken of the three directions of the magnetic components is displayed on a time waveform so that virtually no difference occurs in the detection position of the magnetic field of the subject under examination and having the effect that changes over time in the magnetic field can be accurately detected. Consequently, a wide range can be set for detecting the position and detection of the subject for measurement by the magnetometer housed in the Dewar""s vessel and measurements can be made of high signal-to-noise (S/N) ratios since large signals can be detected. There are virtually no differences in the time waveform due to differences in the detection position of the magnetic field of the subject for measurement, and a time waveform with nearly the same magnetic field can be obtained in a wide region for the subject being measured so that when for instance, the essential subject for measurement is the fetus inside the body of the mother, then variations in the magnetic signal detected due to changes in the positional relationship can be reduced even if changes occur in the positional relationship of mother and fetus. Accordingly, the influence of changes in the magnetic signal due to the detection position of the magnetic field are small and the status of the heart of the fetus can be accurately determined. Also, by passing the signal of each magnetic component of the three directions through a high pass filter, the time required of the detected magnetic signal to stabilize is shortened and measurement can be speeded up since the point where the magnetic signal is greatest can be detected within a short time.
A summary of this invention is described next. A magnetic field measurement apparatus has a plurality of magnetometers comprised of a superconductive quantum interferometer device and detection coils for detecting the magnetic components (BX, BY, BZ) of the three intersecting directions of the magnetic field generated by the subject for measurement, a display means for displaying a time waveform of the magnetic field intensity synthesized from the magnetic field components detected by the magnetometers, a holding means for maintaining the Dewar""s vessel housing the magnetometers, a control means for controlling the positional relation of the Dewar""s vessel and the subject for measurement, and a display means for displaying changes over time in the intensity (magnitude) {square root over ( )}(BX2+BZ2+BZ2) of the magnetic field vector expressing a synthesis of results found from taking the squared sum of the magnetic components of the three directions. Thus no effects are sustained from positional changes of the subject for measurement, the magnetic components of the three directions formed by the electrical current source of the subject for measurement are measured simultaneously, and accurate changes over time in a magnetic field can be detected.
The outline of this invention is as follows. A magnetic field measurement apparatus comprises a plurality of magnetometers each comprising SQUID""s and three detection coils each of which detects each of three orthogonal directional magnetic field components (BX, BY, BZ) of a magnetic field generated from a subject to be inspected, a display which displays time variation of wave form of magnitude {{square root over ( )}(BX2, BY2, BZ2)} of magnetic field synthesized by square sum of each of the three orthogonal directional magnetic field components of the magnetic field generated from the subject to be inspected, a holding means for holding the Dewar""s vessel for arranging magnetometers therein, and a controlling means for controlling a positional relationship between the subject to be inspected and the Dewar""s vessel. Accurate time variation of a magnetic field generated from the subject to be inspected can be detected without influence of positional change of the subject to be inspected, by simultaneously measuring each of three orthogonal directional magnetic field components (BX, BY, BZ) of a magnetic field generated from current sources in the subject to be inspected.